Typically, amine oxides are produced by combining a tertiary amine and hydrogen peroxide in the presence of water. The typical prior art process, whether or not a reaction promoter is employed, is an isothermal process wherein the amine and water, and promoter if used, are heated before addition of peroxide in order to initiate the reaction. Because the reaction is exothermic, once the reaction has been initiated by heat input, the reaction mixture must be cooled to control the temperature of the reaction. After the majority of the reaction is completed, additional heat must be added to the system to achieve a final conversion.
When the typical prior art amine oxide process is carried out at temperatures above 60.degree. C., significant amounts of nitrosamines are produced. Nitrosamine impurities in amine oxides have long been regarded as harmful impurities by most in the surfactant industry because of suspected carcinogenic and mutagenic properties. See for example "Nitrosamines: Assessing the Relative Risk" in Chemical & Engineering News, pages 20-26, Mar. 31, 1980. Nevertheless, according to U.S. Pat. No. 5,498,791 (Mar. 12, 1996), commercial amine oxides contain between 200-1000 ppb nitrosamines.
Heretofore various attempts have been made to reduce the levels of nitrosamines in amine oxides. According to U.S. Pat. No. 5,223,644 (Jun. 29, 1993): "The method generally recommended in the prior art for reducing the nitrosamine levels has been to carry out the reaction at relatively low temperatures, e.g., below 40.degree. C., using the carbon dioxide catalyst to maintain adequate reaction rates. We have found in practice, however, that this approach is not generally effective to produce amine oxides with the desired low nitrosamine content, especially when excess of hydrogen peroxide is used, as has been normal practice to avoid products contaminated with substantial residual unreacted amine." It appears that the prior art referred to in the foregoing quotation is EP 307 184 A2, published Mar. 15, 1989, wherein it is pointed out that tert-amine oxides that are substantially free of nitrosamine impurities can be made by reacting the desired tert-amines with aqueous hydrogen peroxide in the presence of a promoter formed from carbon dioxide if the reaction is conducted at a temperature of 45.degree. C. or lower and preferably below 40.degree. C.
In U.S. Pat. No. 5,223,644, it is proposed to inhibit nitrosamine formation in tertiary amines to levels below 100 ppb and preferably below 50 ppb by including in the tert-amine/hydrogen peroxide reaction mixture or in the resultant tert-amine oxide greater than about 2.5% and up to 20% by weight of the tert-amine of a bicarbonate or carbonate such as an alkali or alkaline earth metal bicarbonate or carbonate, notably sodium bicarbonate.
U.S. Pat. Nos. 5,442,113 (Aug. 15, 1995) and 5,498,791 (Mar. 12, 1996), acknowledge that the use of such relatively high concentrations of carbonates and/or bicarbonates (greater than 2.5% by weight of the amine) to inhibit the formation of nitrosamines "may result in levels of inorganic impurity in the product which are unacceptable to some customers." The approach taken in these two patents is to employ a synergistic mixture of alkali or alkaline earth metal bicarbonate or carbonate such as sodium bicarbonate, with a phosphonate such as an organoamino methylene phosphonate.
From the information presently available, it appears that no previously known process for producing tertiary amine oxides on an industrial scale produced or had the capability of producing a product having both a very low content of nitrosamine and a very low content of free amine. For example, replication of several Examples of U.S. Pat. No. 5,223,644 using a commercially available mixture of dimethyldodecylamine and dimethyltetradecylamine (ADMA.RTM. amine; Albemarle Corporation) in lieu of lauryl myristyl amine produced tertiary amine oxide products that had low nitrosamine contents but which had substantial quantities of free amine (almost 3 wt %). Tertiary amine oxide product produced by Albemarle Corporation heretofore did achieve low levels of nitrosamine, but free amine levels were typically 0.5 wt %. Unfortunately, free amine has a deleterious effect upon the stability of hypochlorite bleaches.
It would be of great advantage if a way could be found of inhibiting formation of nitrosamines during production of tert-amine oxides by the oxidation of tert-amine with hydrogen peroxide without need for addition of metal or phosphorus compounds and consequent contamination of the product with such impurities, especially if the reaction could be safely carried out using temperatures above 40-45.degree. C. It would also be of great advantage if it were possible to provide a tertiary amine oxide product that has very low contents of both nitrosamine and free amine. This invention is deemed to fulfill these needs in a highly effective manner.